1. Field of the Invention
The present disclosure relates to optics, and more particularly to electrically conductive coatings for broadband optics.
2. Description of Related Art
Electro-optic (EO) systems require windows to protect the sensor and electronics from outside elements. In addition to rain, dust, and the like, in many cases the window must also block electromagnetic interference (EMI) that would otherwise impede the EO system performance.
EMI shielding can be accomplished with a window that is electrically conductive and optically transparent. There are three conventional types of shielding.
The first type of EMI shielded window uses a semiconductor material such as silicon or germanium that is doped with a group V element such as phosphorous, arsine, or antimony to supply additional electrons to provide electrical conductivity. These windows are opaque for visible wavelengths and are thus not useful for broadband EO systems.
The second type of shielded window uses a continuous, transparent, conductive coating. These coatings consist of wide bandgap semiconductors such as indium oxide (In2O3) and zinc oxide (ZnO) that have broadband optical transparency. The semiconductors are doped to provide electrical conductivity. However, as doping increases to increase electrical conductivity and EMI attenuation, optical transmittance decreases. This effect begins at longer wavelengths where both plasma reflectance and free-carrier absorption from electrons decrease transmittance. Traditional transparent, conductive semiconductor coatings are practical only in the 0.4 to 2.0 micron range, short wavelength visible through short wavelength infrared, (SWIR).
The third type of shielded window is traditionally required for broadband applications from the visible to the long-wave infrared (LWIR). A grid of fine metal lines is applied on the surface of the window. Typical dimensions are 5-micron wide lines with 140 micron spacing. These gridded windows enable optical transmittance over a broad wavelength range, but they limit optical transmittance by obscuration and scattering.
U.S. Pat. No. 9,276,034 presents a method for reducing the optical scattering from a conductive grid. Channels are etched into a window substrate, and an electrically conductive semiconductor is deposited in the channels such that the surface of the window is planar. The semiconductor is transparent for visible and short wavelength infrared (SWIR) wavelengths but reflecting and absorbing for mid wavelength infrared (MWIR) and longer wavelengths. Using a semiconductor with an index of refraction close to that of the substrate minimizes light scattering from the grid lines.
The conventional techniques have been considered satisfactory for their intended purpose. However, there is an ever present need for improved electrically conductive optical coatings for broad band optics. This disclosure provides a solution for this problem.